Bar codes have been used in a wide variety of applications as a source for information. Typically bar codes are used at a point-of-sale terminal in merchandising for pricing and inventory control. Bar codes are also used in controlling personnel access systems, mailing systems, and in manufacturing for work-in process and inventory control systems, etc. The bar codes themselves represent alphanumeric characters by series of adjacent stripes of various widths, i.e. the universal product code.
A bar code is a set of binary numbers. It consists of black bars and white spaces. A wide black bar space signifies a one and a thin black bar or space signifies a zero. The binary numbers stand for decimal numbers or letters. There are several different kinds of bar codes. In each one, a number, letter or other character is formed by a certain number or bars and spaces.
Bar code reading systems or scanners have been developed to read bar codes. The bar code may be read by having a light beam translated across the bar code and a portion of the light illuminating the bar code is reflected and collected by a scanner. The intensity of the reflected light is proportional to the reflectance of the area illuminated by the light beam. Thus, the scanners read the difference between the light and dark bars by reading the absences of reflected light This light is converted into an electric current signal and then the signal is decoded.
Conventional bar codes are limited in the amount of information they contain. Even two dimensional bar codes such as PDF-417 and Code-1 are limited to a few thousand bytes of information. The ability to encode greater information density is limited by the resolution of available scanning devices.
The prior art has attempted to use colored bar codes to convey additional information. However, color printing is inherently analog and the is fastness, reproducibility and selective delectability of colored bar code imprints as well as the impractically of reproducibly calibrating detection systems, prohibit their use for the digital encoding of additional information.
Bar codes have been affixed to many different types of documents, so that they may be read by a machine, thereby reducing labor costs. Documents that include bar codes have been issued by governmental agencies, financial institutions, brokerage houses, etc., that authorize the holder of such documents to perform authorized tasks or grant rights to the holder of such a document. Examples of such documents are drivers licenses, entry access badges, identification cards, etc. In issuing such documents, it is desirable to have them of a convenient size, while including information necessary for identifying the holder of the document and the rights conferred. Thus, oftentimes, there is not enough room to include the bar code with all of the information one would want to include in the bar code.
Another problem encountered by the prior art when bar codes were affixed to documents is that the bar codes were not to difficult to forge and could be easily copied, hence there was unauthorized use of the documents to which the bar codes were affixed.